/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        if (!head || !head->next)
            return head;
        ListNode* slow = head;
        ListNode* fast = head->next;
        ListNode* prev = nullptr;
        ListNode* newHead = head->next;
        
        while (fast)
        {
            ListNode* Next = fast->next;
            fast->next = slow;
            slow->next = Next;
            if (prev)
                prev->next = fast;

            prev = slow;
            slow = Next;
            if (!slow)
                break;
            fast = slow->next;
        }

        return newHead;
    }
};